1. Field of the Invention
The resent invention relates to a power transfer apparatus for four-wheel drive automotive vehicles, and, more particularly, to a differential power transfer apparatus which has a primary shaft to which engine output is transmitted and a secondary shaft, operationally coupled to the primary shaft, which is coupled to a propeller shaft through a splined sliding yoke.
2. Description of Related Art
Typically, four-wheel drive automotive vehicles are provided with power transfer apparatuses having a primary shaft and a secondary shaft, which are arranged in parallel with each other, for providing driving force to both front and rear propeller shafts. In general, if such a transfer is installed on front-engine rear-drive (FR) vehicles, these primary and secondary shafts are respectively joined to the front and rear propeller shafts. In particular, through the front propeller shaft, the secondary shaft is linked with a front differential. In this instance, since the center axis of rotation of the secondary shaft is ordinarily offset both vertically and horizontally from the axis of the front differential, they are necessarily joined to the front propeller shaft by means of yokes. Further, since the front differential tosses responding to vertical motion of suspensions while the vehicle is running, there is always caused changes in distance between the transfer and the front differential. Various efforts have been made to cancel such a change in order to provide for an optimum arrangement of the drive line.
One such effort is described in Japanese Unexamined Utility Model Publication No. 2-82628. The approach used was to provide a damper mechanism in the drive line between the secondary shaft and the front differential. This damper mechanism includes a splined sliding yoke for providing a splined joint between the secondary shaft and the front propeller shaft so as to cancel changes in distance between the transfer and the front differential.
While the drive line in which the secondary shaft and the front propeller shaft are joined by means of a splined sliding yoke may have advantages over the prior art, nevertheless, various intricacy must be imposed upon the joint and other relating elements. For instance, since the secondary shaft and the front propeller shaft intersect at an angle, these shafts have a slight bend between them at the splined joint, causing swaying engagement between them. When a bend changes increasingly or decreasingly while these shafts rotate and slide relatively each other, vibrations and noise are generated which are always undesirable. In this instance, since many transfers have secondary shafts which are relatively light in weight and have natural frequencies as high as those responsive to normal rotational speeds of engines, vibration accompanying with slide displacement of the splined joint is amplified due to resonance through the secondary shaft, making noise and sound.
In addition, the power transfer apparatus includes a power transmission means having a drive sprocket fixedly mounted on the primary shaft, a driven sprocket mounted for rotation on the secondary shaft, and a drive chain engaging these sprockets so as to transmit driving power to a driving power transfer means, such as a fluid coupling, mounted on the secondary shaft. In order to allow smooth differential motion between the first and second shafts, it is necessary to lubricate the interface between the rotary member and the secondary shaft.
Lubrication of the interface between these rotary member and secondary shaft is typically made with oil in the transfer casing. However, because the drive chain scoops up the lubrication oil and splashes it, there is caused shortage of the lubrication oil in the transfer casing, leading to an insufficient lubrication of the interface.